It looks like the U.S. stock market will finally get something that happens, on average, about once a year: a 10+% percent drop—the definition of a market correction. The last time this happened was a whopper—the Great Recession drop that caused U.S. stocks to drop more than 50% - so most people today probably think corrections are catastrophic. They aren’t. More typically, they last anywhere from 20 trading days (the 1997 correction, down 10.8%) to 104 days (the 2002-2003 correction, down 14.7%). Corrections are unnerving, but they’re a healthy part of the economy—for a couple of reasons.

Reason #1: Because corrections happen so frequently and are so unnerving to the average investor, they “force” the stock market to be more generous than alternative investments. People buy stocks at earnings multiples which are designed to generate average future returns considerably higher than, say, cash or municipal bonds—and investors require that “risk premium” (which is what economists call it) to get on that ride. If you’re going to take more risk, you should expect at least the opportunity to get considerably more reward.

Reason #2: The stock market roller coaster is too unsettling for some investors, who sell when they experience a market lurch. This gives long-term investors a valuable—and frequent—opportunity to buy stocks on sale. That, in turn, lowers the average cost of the stocks in your portfolio, which can be a boost to your long-term returns.

The current market downturnrelates directly to the first reason, where you can see that bonds and stocks are always competing with each other. Monday’s 4.1% decline in the S&P 500 coincided with an equally-remarkable rise in the yields on U.S. Treasury bonds. Treasuries with a 10-year maturity are now providing yields of 2.85%--hardly generous, but well above the record lows that investors were getting just 18 months ago. People who believe they can get a decent, relatively risk-free return from bond investments are tempted to abandon the bumpy ride provided by stocks for a smoother course that involves clipping coupons. Bond rates go up and the very delicate supply/demand balance shifts, at least temporarily, in their direction, and you have the recipe for a stock market correction.